1. Field of the Invention
The present invention relates to an antenna unit which is formed by combining a radiator having low conductor loss and a printed circuit board with each other, and more particularly, it relates to an antenna unit which is suitably applied to a mobile communication device, for example.
2. Description of the Background Art
An antenna unit must have excellent characteristics such as high gain and low reflection loss, and further miniaturization is required for an antenna which is applied to a mobile communication device.
In general, an inverted-F antenna unit is known to be usable as a miniature antenna in a mobile communication device.
An example of such an inverted-F antenna is described in "Small Antennas" by F. Fujimoto, A. Henderson, K. Hirasawa and J. R. James, Research Studies Press Ltd., England.
An exemplary inverted-F antenna unit is now described with reference to FIG. 1. Referring to FIG. 1, an inverted-F antenna unit 1 has a ground plate 2 which is connected to an earth potential, and a radiating plate 3, consisting of a metal plate, which is arranged above the ground plate 2 in parallel with the same. The radiating plate 3 is adapted to radiate electric waves. A short pin 4 is integrally formed on the radiating plate 3, to extend from its side edge toward the ground plate 2. This short pin 4 is electrically connected to the ground plate 2. Thus, the radiating plate 3 is shorted with respect to the ground plate 2 by the short pin 4. The ground plate 2 is provided with a coaxial cable connecting part 2a, which is connected with a coaxial cable or a coaxial connector for feeding the radiating plate 3.
While FIG. 1 typically illustrates the inverted-F antenna unit, it is necessary to provide the coaxial connecting part 2a on a printed circuit board for forming the coaxial connector or cable connecting part 2a in order to form a ground electrode on the printed circuit board thereby structuring the aforementioned ground plate 2 in practice. Further, it is necessary to connect a coaxial connector or a coaxial cable to the coaxial connecting part 2a which is formed on the printed circuit board. Thus, the coaxial connector or the coaxial cable must inevitably project from a major surface of the printed circuit board which is opposite to that provided with the antenna unit 1, which seriously hinders further miniaturization of the antenna unit 1.
In the inverted-F antenna unit 1, further, its gain is varied with the size of the radiating plate 3. In other words, the gain of the inverted-F antenna unit 1 is reduced as the size of the radiating plate 3 is reduced. When the radiating plate 3 is sized to be not more than about 1/10 of the wavelength of its resonance frequency in the inverted-F antenna unit 1 which is applied to a mobile communication device, it is impossible to attain a sufficient gain. In other words, it is extremely difficult to implement a miniature antenna unit having a high gain with the conventional inverted-F antenna unit 1.